High-temperature material transferring member

ABSTRACT

A high-temperature material transferring member including a coating film formed on a surface of base metal, wherein the coating film is a composite coating film using a mixed powder made up of: a Co-based alloy powder containing, in mass %, 0.03 to 0.6% of C, 0.2 to 3% of Si, 22 to 35% of Cr, and more than 50% of Co; and a Cr carbide powder, the composite coating film is formed by the plasma powder overlaying process. The high-temperature material transferring member has excellent build-up resistance particularly in a gas atmosphere of 1100° C. or more. The high-temperature material transferring member has excellent in build-up resistance, oxidation resistance, and heat cracking resistance.

The disclosure of International Application No. PCT/JP2009/071628 filedDec. 25, 2009 including specification, drawings and claims isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a high-temperature materialtransferring member suitable for retaining and transferring ahigh-temperature material, such as steel materials, heated by heattreatment etc., and in particular to a high-temperature materialtransferring member such as transfer rollers and skid buttons in a heattreatment furnace.

BACKGROUND ART

In hot working of metal materials such as hot processing for theproduction of seamless tubes and the like, it is common to usetransferring members such as roller conveyors for transferring, forexample, materials to be processed or products in a heat treatmentfurnace, or those materials heated to high temperatures in a heattreatment furnace, etc. (hereinafter, generally referred to as“high-temperature materials”).

If for example slipping occurs between high-temperature material and thetransferring member when the high-temperature materials are beingtransferred, sticking may occur between the high-temperature materialand the transferring member. At that moment, if peeled-off material fromthe high-temperature material or oxide thereof locally adheres to thehigh-temperature material or the transferring member surface(hereinafter, such deposits are referred to as “build-ups”), a problemarises in that an indentation defect occurs on the surface of thehigh-temperature material being transferred, thus deteriorating thesurface quality and yields of the product.

In order to suppress such defects, attempts have been made so far, forexample, to select a material having excellent sticking resistance suchas a Cr or Ni alloy as the base metal of the transferring member;however, such attempts alone have not achieved sufficient effects inreality. Accordingly, as shown below, various members have been proposedin which a coating film is formed on the surface of base metal.

For example, Patent Document 1 discloses an invention for providing astrong oxide-scale coating film on the surface of the base metal of aroller by performing overlay welding on the surface of the base metalwith a heat resistant material based on 3Cr-1Ni—Fe alloy containing 30to 50 vol % of NbC, and then subjecting the coating film to a heattreatment in an oxidizing atmosphere containing CO gas. According tothis invention, it is stated that a hot transfer roller having excellentwear resistance and sticking resistance can be produced, whereby noadhesion will occur between the two surfaces of hot processing materialand roller when transferring the hot processing material.

Patent Document 2 discloses a technique for preventing the sticking of ahot processing tool, in which the hot processing tool has a two-layercoating film made up of a metal-carbide composite coating filmcontaining Nb carbide particles in volume ratio of 20 to 70%, and anoxide coating film formed on the outermost surface of the concernedcoating film, on the surface of base metal. Moreover, Patent Document 3proposes an invention relating to a composite welding material forplasma powder overlay welding made up of an alloy powder consisting ofCr, W, Fe, and C, the balance being Co, and a carbide-based ceramicpowder of 20 to 70 wt %, and Patent Document 4 proposes an inventionrelating to a roll overlaid with cobalt or a cobalt-based alloycontaining 20 to 60 vol % of chromium carbide.

Patent Document 5 discloses an invention relating to a surface treatmentmethod for metal parts, in which powder of Co—Cr—Fe alloy material addedwith Cr₃C₂ is overlaid by a plasma powder welding to form a lininglayer. Patent Document 6 discloses an invention relating to ahigh-temperature material transfer roller having excellent build-upresistance, in which a metal-carbide composite coating film containing20 to 70 vol % of carbides with the balance being metal is formed on theoutermost surface of the roller. Further, Patent Document 7 proposes amethod in which to suppress the occurrence of cracking in a hard-facedoverlay layer, a molten carbon steel, which is added with hard particlesof carbide or carbo-nitride in an unmelted state with part of them beingmelted, is solidified and is further quenched. Patent Documents 8 and 9disclose inventions relating to welding materials for overlay, whichrespectively contain 25.0 to 45.0%, and 20 to 40% of Co with the balancebeing Fe.

Besides the methods for forming overlay layer as described above, therehave been proposed technologies relating to the surface modification byuse of a spraying process.

For example, Patent Document 10 discloses an invention relating to asprayed roll for processing steel materials, in which a sprayed coatingfilm made up of 10 to 50 area % of hard particles having a graindiameter of 1 to 100 μm, and 90 to 50 area % of matrix alloy phase isprovided on the surface of a barrel portion of the roll. Moreover,Patent Document 11 discloses an invention relating to an electrolyticplating conductor roll, in which reprecipitated carbides are dispersedin a sprayed coating film of mixed powder made up of 10 to 60 wt % ofcarbide cermet powder and 90 to 40 wt % of C-containing nickel chromiumalloy powder.

Patent Document 12 discloses an invention relating to a roll in a heattreatment furnace, having a carbide-based coating film excellent inbuild-up resistance, heat resistance, and wear resistance, in which acermet material of an alloy containing 50 to 90 wt % of chromiumcarbide, with the balance being one or two kinds of nickel and cobalt,and one or two kinds of chromium and aluminum is sprayed on the surfaceof the roll. Further, Patent Document 13 discloses an invention relatingto a sprayed coating film suitable for a sliding wear member subjectedto repeated thermal shocks, the spray coating film made up of a heatresistant alloy containing 5 to 30 wt % of one or more kinds of carbide,boride, oxide, and composite oxide of Cr etc., with the balance beingCo, Cr and Mo.

For use with a transfer roll in a heat treatment furnace for steelstrips, Patent Document 14 discloses an invention relating to a cermetpowder for spray coating excellent in build-up resistance, in which thecermet powder is made up of an alloy powder containing Al, Cr and Y,with the balance being one or more kinds of Co and Ni, and a ceramicpowder including one or more kinds of boride and carbide. Moreover,Patent Document 15 discloses an invention relating to a roll forcontinuous casting, which is formed with 0.5 to 3 mm of a spray coatedlayer made up of a self-influxing alloy (Ni-based or Co-based)containing 10 to 50 wt % of carbide such as tungsten carbide, chromiumcarbide, and niobium carbide, or a mixture thereof bonded with one ormore metal binders, the balance being C: 0.02 to 0.25 wt %, and Cr: 0.5to 15 wt %. Furthermore, Patent Document 16 discloses an inventionrelating to a roll for transferring high-temperature steel plate, inwhich the roll is coated, on the surface of the base metal, with a mixedcoating layer of ceramics and nickel-based alloy or cobalt-basedself-influxing alloy by a spraying process.

LIST OF PRIOR ART DOCUMENTS Patent Literature

-   [Patent Document 1] JP5-84570A-   [Patent Document 2] JP6-315704A-   [Patent Document 3] JP64-18599A-   [Patent Document 4] JP3-207510A-   [Patent Document 5] JP8-13116A-   [Patent Document 6] JP2003-340511A-   [Patent Document 7] JP2008-763A-   [Patent Document 8] JP62-134193A-   [Patent Document 9] JP64-11093A-   [Patent Document 10] JP3-2362A-   [Patent Document 11] JP5-295592A-   [Patent Document 12] JP6-33149A-   [Patent Document 13] JP9-316621A-   [Patent Document 14] WO01/34866-   [Patent Document 15] JP2006-263807A-   [Patent Document 16] JP63-255352A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The technologies proposed by Patent Documents 1 and 2 relate to theformation of composite coating films of NbC and an alloy on the basemetal, and are intended for the prevention of sticking. However, NbC haspoor oxidation resistance, and it cannot stably exist particularly in ahigh-temperature gas atmosphere, thereby exhibiting a sharp decline inwear resistance. The technologies proposed by Patent Documents 3 to 5relate to the formation of composite coating films of various carbidesincluding Cr carbide and a metal on the base metal, and are intended forthe improvement of wear resistance at high temperatures. However, as theresult of having tried to improve wear resistance, these coating filmsbecome to have a hard and brittle metal micro-structure so that crackingbecomes more likely to occur during cooling in the working of plasmapowder overlay. Since wear resistance declines in a crack portion, thesecoating films have a problem that uniform quality cannot be obtained.Moreover, as the result of being subjected to thermal cycles of heatingand cooling during a use for many hours, further cracking occurs, thusimpairing wear resistance.

Since the coating film proposed by Patent Document 6 also becomes tohave a brittle metal micro-structure having a high hardness, it isdifficult to avoid the occurrence of cracking, and the deposition ofoxide becomes easier around a crack portion so that build-up resistancedeteriorates. Further, while the invention described in Patent Document7 intends to suppress the occurrence of cracking in a hard and brittleoverlay layer, since it is a coating film consisting of a fillermaterial made up of carbon steel and carbide, enough oxidationresistance cannot be obtained so that the surface of the coating filmchanges, and wear resistance and build-up resistance deteriorate aswell. On the other hand, in the inventions described in Patent Documents8 and 9, since the welding material is made up of a reduced content ofCo, with the balance being Fe, although it is possible to suppress theoccurrence of cracking in the overlay layer, wear resistance is notsufficient.

In the methods for forming a coating film by spraying shown in PatentDocuments 10 to 16, the coating film is formed by converting a coatingmaterial, which is heated and melted, into fine particles by gas flow,and making it collide with and stack on the base material. Therefore, ingeneral, pores are likely to be formed in the coating film, andtherefore it cannot be said that wear resistance and build-up resistanceare sufficient. Moreover, while the coating film formed by a plasmapowder overlaying process is melted to be bonded (chemically bonded)with the base material, a sprayed coating film and the base material arephysically bonded, and therefore the bonding strength thereof is weak.Therefore, in a transfer roller for high-temperature material, a formedcoating film peels off while in use, thus sharply reducing wearresistance and build-up resistance. Since the mixed coating layer shownin Patent Document 16 has a low Cr content, oxidation resistance is notsufficient, and not suitable particularly for a transfer roll forhigh-temperature materials, particularly reaching a temperature of 1100°C. or more.

The present invention has its object to provide a high-temperaturematerial transferring member, which is excellent in build-up resistance,wear resistance, and oxidation resistance in a high-temperature gasatmosphere, and in which the occurrence of cracking during plasma powderoverlay working is suppressed, and more specifically, a high-temperaturematerial transferring member, which has excellent build-up resistancewhen used in a high-temperature gas atmosphere, in particular, a gasatmosphere of high temperature reaching 1100° C. or more, and in whichthe occurrence of indentation defect is prevented.

Means for Solving the Problems

The present inventors have conducted various studies on the method forpreventing a build-up, which is effective even in the transfer byrollers in a gas atmosphere which becomes a high temperature,particularly 1100° C. or more, such as in a heat treatment furnace, andhave consequently obtained the following findings.

(a) Conventional methods for preventing build-ups have placed anemphasis on the prevention of sticking and adhesion between ahigh-temperature material and a transferring member. For this reason,they have been looking to utilize hot lubricating effect by oxide scaleof the surface of the transferring member and enhance the effectthereof; however, such a prevention method has its limitation insuppressing the occurrence of indentation defect by build-ups. That is,so far, emphasis has been placed on strengthening the oxide scale, whichhas a hot lubricating effect, to make it resistant to peeling off;however, the oxide scale becomes thin due to usage for a long period oftime, or peels off due to impact, and it has not been possible tosuppress build-ups.

(b) Accordingly, the present inventors focused attention on that inorder to effectively suppress the occurrence of indentation defects inthe surface of the high-temperature material being transferred, it iseffective to form an overlay coating film on the surface of thetransferring member, and to reduce the adhesiveness of oxide scale (A)which is produced when the coating film is oxidized, that is, toincrease peelability. With such a configuration, even if the oxide scale(B) of the transferred material side is migrated to the oxide scale (A)of the transferring member side, the oxide scale (A) easily peels offfrom the overlay coating film, and therefore the oxide scale (B) will beremoved together with the oxide scale (A). As a result, it is possibleto prevent build-ups, and effectively suppress the occurrence of surfacedefects such as indentation defects on the high-temperature material tobe transferred.

(c) It has been found that to increase the peelability of the oxidescale (A) on the overlay coating film, it is effective to dispersedifferent materials that reduce adhesiveness in the overlay coatingfilm. After conducting various studies on the pertinent differentmaterials, it has been found that Cr carbides suppress build-upresistance. Such effects are particularly remarkable in a gas atmosphereof a temperature of 1100° C. or more. Moreover, it has been found thatproviding an alloy-Cr carbide composite coating film (hereinafter,simply referred to as a “composite coating film”), in which Cr carbideis dispersed, can provide wear resistance.

(d) As a result of a study on the application of the composite coatingfilm in a high-temperature gas atmosphere, it has been found thatbesides the build-up resistance and wear resistance, oxidationresistance is also important for long-term usage. Further, it has beenfound that cracking may occur in the working of plasma powder overlay,and such a defect will impair build-up resistance and wear resistance.Having investigated the effects of the components of alloy powder of thecomposite coating film to solve the problems described above, it hasbeen found that it is important to have Cr and Si being contained by adesired amount to improve oxidation resistance, and on the other hand,it is important to reduce the hardness of the composite coating film bylimiting the C content to suppress the occurrence of cracking.

(e) Meanwhile, although it is possible to suppress the occurrence ofcracking in the composite coating film during overlay working bylimiting the C amount of the alloy powder, a problem exists, on theother hand, in that wear resistance is likely to decline. For thatreason, it is necessary to appropriately control the C content. Further,it has been found that to secure wear resistance, it is essential tomake the principal component of the alloy powder be Co, instead of Ni orFe. That is, a composite coating film using a Co-based alloy powdercontaining Co of more than 50%, with C content being 0.03 to 0.6 mass %,can realize wear resistance and suppression of cracking during overlayworking at the same time, and can exhibit excellent build-up resistance.In the transferring member that satisfies the conditions describedabove, even if a scale of high-temperature material such as a startingmaterial for a seamless tube is melted and adheres to the surface of thetransferring member causing a build-up, it will peel off and fall outbefore it grows, and therefore the build-up will not grow large. As aresult of this, it is possible to transfer high-temperature materialsfor a long period of time without causing surface defects thereon.

According to the findings (b) to (e), an excellent high-temperaturematerial transferring member can be obtained. However, the presentinventors have found as a result of further research that wearresistance and build-up resistance may decline depending on useconditions, and cracking may occur during the working of plasma powderoverlay. As the result of carefully analyzing the main factors thereof,the following findings have been obtained.

(f) It has been found that the occurrence of cracking is also affectedby the volume ratio of Cr carbide in the composite coating film. Thatis, when the content rate of Cr carbide is too small, build-upresistance and wear resistance may not be secured, and on the otherhand, when the content ratio is too large, a large number of cracks mayoccur during working. For that reason, it has been found that thecontent of Cr carbide powder is preferably 20 to 70 vol % in volumeratio with respect to the total amount of mixed powder.

(g) This phenomenon occurred only when used in a temperature range,particularly exceeding 1200° C., and was a phenomenon peculiar to a useenvironment of ultra-high temperature. Then, an investigation on thereactivity of the composite coating film was conducted by using DTA(differential thermal analysis), and an endothermic reaction is observednear the use temperature. Although details of this reaction are notfully clear, it is likely that a state change where the compositecoating film starts fusing, or a chemical change of carbides hasoccurred. As the result of the occurrence of such a reaction, thestrength of the composite coating film sharply declines, resulting indeclines of wear resistance and build-up resistance thereof.Investigation on the effects of elements in the alloy powder has foundthat Si, which is contained to increase oxidation resistance of Co-basedalloy, and C, which is contained to increase wear resistance, both lowerthe temperature at which endothermic reaction occurs (hereinafter,referred to as an “endothermic reaction temperature”). Accordingly, itis necessary to appropriately control Si and C contents. Moreover, whenconsidering operation at 1200 to 1250° C., the micro-structure of thecomposite coating film is preferably controlled such that theendothermic reaction temperature observed in the composite coating filmis 1250° C. or more.

The present invention has been made based on such findings, and its gistis high-temperature material transferring members shown in (1) to (6)described below.

(1) A high-temperature material transferring member including a coatingfilm formed on a surface of base metal by a plasma powder overlayingprocess, wherein the coating film is a composite coating film using amixed powder made up of: a Co-based alloy powder containing, in mass %,0.03 to 0.6% of C, 0.2 to 3% of Si, 22 to 35% of Cr, and more than 50%of Co; and a Cr carbide powder.

(2) A high-temperature material transferring member including a coatingfilm formed on a surface of base metal by a plasma powder overlayingprocess, wherein the coating film is a composite coating film using amixed powder made up of: a Co-based alloy powder containing, in mass %,0.03 to 0.6% of C, 0.2 to 3% of Si, 22 to 35% of Cr, and more than 50%of Co, with the balance being impurities; and a Cr carbide powder.

(3) The high-temperature material transferring member according to theabove (2), wherein the Co-based alloy powder further contains, in mass%, one or more elements selected from the following groups <1> to <4>:

<1> Mn: 10% or less, Cu: 10% or less, Ni: 10% or less, and Fe: 10% orless;

<2> Mo: 10% or less, and W: 10% or less;

<3> B: 3% or less, Ti: 3% or less, V: 3% or less, Zr: 3% or less, Nb: 3%or less, and Ta: 3% or less; and

<4> Al: 1% or less, Ca: 1% or less, and REM: 1% or less.

(4) The high-temperature material transferring member according to anyof the above (1) to (3), wherein the content of Cr carbide powder is 20to 70 vol % in a volume ratio with respect to the total amount of mixedpowder.

(5) The high-temperature material transferring member according to anyof the above (1) to (4), wherein an endothermic reaction temperature ofthe coating film is 1250° C. or more.

(6) The high-temperature material transferring member according to anyof the above (1) to (5), wherein the high-temperature materialtransferring member has excellent build-up resistance in a gasatmosphere of 1100° C. or more.

Advantageous Effects of the Invention

According to the present invention, even if a high-temperature materialis transferred under a high-temperature environment such as inside aheat treatment furnace, no build-up will be produced for a long periodof time, and thus no indentation defect on the transferred material willoccur. Further, it also provides an excellent durability. Thus, sincethe present invention contributes to quality improvement of hot workingproducts, increase of yields, as well as reduction of production cost byextending the life of transfer rollers, advantageous effects thereof areremarkable. In particular, since as the contact area between thematerial to be transferred and the roll decreases, the pressure at thecontact portion increases, resulting in a pronounced build-up and wear,the present invention is suitable for rolls for use in transferringmetal materials having a small contact area such as tube-shapedmaterials.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional schematic view to show the configuration ofa rotary friction compressing tester.

MODE FOR CARRYING OUT THE INVENTION

A high-temperature transferring member relating to the present inventionhas a composite coating film of Co-based alloy-Cr carbide on the surfaceof the base metal thereof. This composite coating film is formed by aplasma powder overlaying process by using a Co-based alloy powder and aCr-carbide powder. In the following description, unless otherwisedefined, “%” regarding the content of a component refers to “mass %”.

1. Co-Based Alloy Powder

C: 0.03 to 0.6%

C has a function of increasing the hardness at high temperatures, and toachieve this effect, its content needs to be 0.03% or more. On the otherhand, if it is contained more than 0.6%, cracking will occur in anoverlay coating film during cooling in the working by plasma powderoverlaying, thereby significantly reducing wear resistance and build-upresistance in the crack portion. Therefore, the content of C isdetermined to be 0.03 to 0.6%. It is noted that since C decreases theendothermic reaction temperature, its upper limit is preferably 0.5%,and a more preferable upper limit is 0.4% particularly for hightemperature use. Moreover, with the C content being 0.05% or more, theeffects described above will become more pronounced. More preferably,the C content is 0.1% or more.

Si: 0.2 to 3%

Si is an element that increases oxidation resistance of the compositecoating film in a high-temperature gas atmosphere, and it needs to becontained 0.2% or more. On the other hand, if Si is contained more than3%, the improvement effect of oxidation resistance is small, and theendothermic reaction temperature of the composite coating film willrather be reduced, thus sharply reducing build-up resistance and wearresistance at high-temperature operations. Therefore, the Si content isdetermined to be 0.2 to 3%. From the viewpoint of oxidation resistance,the content is preferably 0.3% or more. On the other hand, in the pointsof build-up resistance and wear resistance, its content is preferably 2%or less. Particularly, when used in a gas atmosphere of 1100° C. ormore, its content is preferably 1% or less.

Cr: 22 to 35%

Cr is an effective element to increase oxidation resistance, and itneeds to be contained 22% or more. On the other hand, if Cr is containedmore than 35%, the composite coating film will become hard and brittle,thus promoting cracking during the overlay working. Moreover, sincetoughness is reduced, cracking will be occurred when the member issubjected to a rapid heating/cooling load during operation. In such acrack portion, build-up resistance and wear resistance will sharplydecline. Therefore, the content of Cr is determined to be 22 to 35%. Apreferable lower limit of Cr content is 23%, and a preferable upperlimit thereof is 33%.

Co: more than 50%

Compared with Ni-based alloys and Fe-based alloys, a Co-based alloy canincrease the hardness at high temperatures and therefore exhibitsadvantageous effects in wear resistance. Therefore, in the presentinvention, it is decided to use a Co-based alloy powder containing morethan 50% of Co as the base component of the composite coating film. Whenemphasis is placed on wear resistance, its content is preferably 55% ormore, and more preferably 61% or more.

Besides the aforementioned elements, the following elements may becontained in an appropriate amount as needed to improve properties suchas high-temperature strength and toughness.

Mn, Cu, Ni, and Fe: 10% or less for each

Since Mn, Cu, Ni, and Fe are effective elements to stabilize themicro-structure, one or more kinds selected from these elements may becontained as needed. However, if they are excessively contained, thewear resistance of composite coating film will decline. Therefore, thecontents of these elements when they are contained are preferably 10% orless, respectively. More preferable upper limit contents are 9%,respectively. It is noted that if the elements described above arecontained even in minute quantities, the effects described above will beachieved; however, the effects will become pronounced when each elementis contained 0.1% or more. Although the total content of these elementsis not specifically designated, it is preferably 20% or less.

Mo and W: 10% or less for each

B, Ti, V, Zr, Nb, and Ta: 3% or less for each

Since Mo, W, B, Ti, V, Zr, Nb, and Ta are effective to improvehigh-temperature strength, one or more kinds selected from theseelements may be contained as needed. However, if they are excessivelycontained, carbides, nitrides, brittle phases precipitate in the alloyand the micro-structure thereof becomes brittle so that cracking is morelikely to occur during the overlay working. Moreover, there is a riskthat cracking occurs due to abrupt stopping and cooling duringhigh-temperature operation. Therefore, when one or more kinds selectedfrom Mo and W are contained, the contents thereof are preferably 10% orless, respectively, and when one or more kinds selected from B, Ti, V,Zr, Nb, and Ta are contained, the contents thereof are preferably 3% orless, respectively. More preferable upper limits are 7% respectively forMo and W, and 1.4% respectively for B, Ti, V, Zr, Nb, and Ta. It isnoted that if the elements described above are contained even in minutequantities, the effects described above will be achieved; however, theeffects will become pronounced when the content is 0.001% or more for B;0.01% or more respectively for Ti, V, Zr, Nb, and Ta; and 0.1% or morerespectively for Mo and W. Although the total content of these elementsis not specifically designated, it is preferably 6% or less.

Al, Ca, and REM: 1% or less, respectively

Since Al, Ca, and REM, which have a strong affinity with oxygen, may becontained for the immobilization of oxygen during the overlay working.However, if they are excessively contained, coarse oxides will beproduced in the alloy, thereby impairing the workability of overlaywelding. Therefore, when one more kinds selected from these elements arecontained, those contents are preferably 1% or less, respectively. TheAl content is more preferably 0.3% or less, and the contents of Ca andREM are more preferably 0.1% or less, respectively. Where, “REM” statedin the present description refers to a general term for a total of 17elements including Sc, Y and lanthanoid etc., and the content of REMrefers to the total content of the elements described above. Moreover,if the elements are contained even in minute quantities, the effectsdescribed above will be achieved; however, the effects will becomepronounced when the content is 0.005% or more for Al, 0.001% or more forCa, and 0.01% or more for REM. Although the total content of theseelements is not specifically designated, it is preferably 2% or less.

It is noted that a Co-based alloy powder may contain elements such as P,S, N, and O as impurities. It is also possible to increase strength byactively containing these elements. Impurities refer to elements whichare introduced from raw materials such as ores and scraps etc., and byother causes when the alloy material is industrially produced, and whichare permitted within a range not adversely affecting the presentinvention.

2. Cr Carbide

Cr carbide has an effect of increasing high-temperature hardness whilesecuring oxidation resistance at high temperatures. Therefore, itexhibits excellent oxidation resistance and wear resistance in acomposite coating film with alloy powder. Particularly, it is effectivein a gas atmosphere of 1100° C. or more. Cr carbide includes Cr₃C₂,Cr₇C₃, Cr₂₃C₆, and so on. Moreover, these carbides may be combined witheach other. Although the content of Cr carbide is not specificallydefined, it is preferably 20 vol % or more in the volume ratio withrespect to the total amount of mixed powder to increase the peelabilityof the oxide scale (A) on the composite coating film of the transferringmember. More preferably, the content is 30 vol % or more. On the otherhand, when the content of Cr carbide exceeds 70 vol %, the effect ofimproving build-up resistance is saturated, and besides the proportionof alloy decreases thereby reducing the retaining power of carbides sothat the formation of composite coating film may become difficult.Moreover, it may promote cracking during the plasma powder overlayworking. For this reason, the content of Cr carbide in the compositecoating film is preferably 70 vol % or less.

Here, although there is no specific limitation on the size of Crcarbide, it is preferable to use carbide particles having an averagediameter within a range of 50 to 200 since those are effective to beuniformly dispersed in the matrix. The shape of carbide may bespherical, elliptic, and rod-like shapes, or a mixture thereof.

3. Method of Forming Composite Coating Film

The method for forming a composite coating film in a high-temperaturematerial transferring member relating to the present invention utilizesa method of feeding the mixed powder described above into a plasmagenerated between an electrode and the base metal to cause the same tobe melted and overlaid on the surface of base metal, that is, a plasmapowder overlaying process. According to this method, smaller porositycompared with a case of plasma spraying etc. and good adhesiveness withthe base material enable the reduction of damages due to peeling off.Further, the method enables to conveniently form the coating film, andis advantageous in the production cost as well.

Although there is no limitation on the thickness of composite coatingfilm, it is preferable that the thickness is 0.3 mm or more to securesufficient surface strength in a range from normal temperatures to hightemperatures. Particularly, when used in a gas atmosphere of 1100° C. ormore, a thickness of 0.5 mm or more is preferable, and a thickness of 1mm or more is more preferable.

4. Endothermic Reaction Temperature

It is important that a composite coating film which has undergone anoverlay working of a mixed powder of Co-based alloy powder and Crcarbide by a plasma powder overlaying process does not change themicro-structure state in association with a reaction at a hightemperature. When the endothermic reaction temperature of the coatingfilm is less than 1250° C., there is a risk that it cannot maintainsufficient build-up resistance and wear resistance as a transferringmember, for example, for a heat treatment furnace of high temperaturesuch as more than 1200° C. to 1250° C. Therefore, the endothermicreaction temperature is preferably 1250° C. or more. It is noted thatthe temperature at which reaction starts is controlled by thecomposition of alloy powder, the kind of hard particles, and the mixingratio thereof of the composite coating film. The endothermic temperaturecan be determined in such a way that a specimen is cut out from thecomposite coating film, electromotive force is measured by for example,TG-DTA (differential thermal balance) etc., and the endothermictemperature is determined from the change of electro motive forceassociated with heat absorption.

5. Base Metal of High-Temperature Material Transferring Member

As the base metal of the high-temperature material transferring memberaccording to the present invention, known steels which have beenconventionally used for transfer rollers may be used. In particular,when used in a high-temperature furnace (in a high-temperatureatmosphere), it is necessary that deformation due to repeated thermalstress, and breakage due to the propagation of a crack will not occur,and the base metal may be appropriately selected taking intoconsideration of the temperature, deformation resistance and useconditions of the material to be processed. For example, stainless caststeel, heat resistant cast steel, and the like may be used. Although theshape of the high-temperature material to be transferred may bearbitrary, the present invention exhibits its effects of build-upresistance and wear resistance particularly when used for the rollersfor transferring metal tubes.

After a composite coating film is formed on the surface of base metal,it can be used as a product as it is. Moreover, stress relief annealingand cutting processing of the outer surface may be appropriately carriedout.

EXAMPLES

First, a mixed powder (Sample Nos. 1 to 14) obtained by mixing an alloypowder having a chemical composition shown in Table 1 and a carbidepowder, and an alloy powder (Sample No. 15) were prepared. Using thesepowders, a coating film of a thickness of 0.7 to 4.5 mm was formed onthe surface of a base metal made up of Ni—Cr alloy by a plasma powderoverlaying process, and used as a sample material. These samplematerials were subjected to the following test to confirm theadvantageous effects of the present invention.

[Build-Up Test]

From each of the sample materials described above, a cylindricalspecimen (test material A) having a diameter of 20 mm and a length of 50mm was cut out such that the surface formed with the coating filmcorresponded to the end surface of the specimen, and on the other hand acylindrical specimen (test material B) having a diameter of 20 mm and alength of 50 mm was cut out from SUS 304 steel, so that both thespecimens were subjected to a build-up test by using a tester shown inFIG. 1.

FIG. 1 is a cross-sectional schematic view to show the configuration ofa rotary friction compressing tester which simulates an occurrence ofbuild-ups. This tester is an experimental apparatus that simulates abuild-up that occurs when a high-temperature material is transferred bya transferring member such as a roller conveyor in a heat treatmentfurnace. In FIG. 1, reference sign 1 denotes a cylindrical test materialA that simulates the high-temperature material transferring member;reference sign 2 denotes a coating film formed on the end surface of thetest material A, reference sign 3 denotes a cylindrical test material Bthat simulates the material to be transferred, reference sign 4 denotesa high-frequency heating coil, reference signs 5 a and 5 b denote testmaterial supports, and reference sign 6 denotes a heating chamber,respectively.

After the test materials were mounted to the supports in the upper andlower parts of the test apparatus, the heating chamber was closed, andthe high-frequency heating coil was energized and kept for one hour sothat the test material A was heated to 1250° C. in the atmosphere. Inthe beginning, the test material B (reference sign 3) was stopped at anupper standby position of the test material A as shown in FIG. 1. Next,the test material B was moved downward while being rotated so that theend surface thereof was brought into pressure contact with the surfaceof the composite coating film of the test material A. At this moment,load P applied to the test material B was 98 N, and the rotational speedwas 5 rpm, and the pressure contact time in one cycle of pressurecontact operation was 6 seconds. After the end of pressure contact, thetest material B was returned to the upper standby position, and thuscompleting one cycle of pressure contact operation. After repeating thispressure contact operation for 100 cycles, the temperature was lowered,and the test material A was dismounted from the support and subjected toan investigation of the occurrence of build-up by visual observation.This operation was repeated until a build-up occurred. The number ofcycles at which a build-up occurred is shown in Table 1.

It is noted that the time from the start of descent of the test member Bto the start of pressure contact, and the time from the end of pressurecontact to the end of ascent were both 15 seconds, the time until theend of 100 cycles was about 1 hour, and thereafter cooling to aroundnormal temperature was performed for about 1 hour. The test material inwhich a built-up occurred after 500 cycles was judged to be excellent inbuilt-up resistance.

[Abrasion Test]

A specimen having a thickness of 5 mm, a width of 25 mm, and a length of50 mm including the coating film of the sample material described abovewas cut out. This specimen was subjected to the investigation of theabrasive wear of the composite coating film using an Ogoshi-typeabrasion tester. The test was conducted by using SUJ-2 as the oppositematerial, at a load of 12.75 kg and a test distance of 200 m, at anormal temperature. The abrasive wear (mm³) after test was obtained andshown in Table 1. It is noted that the sample material in which anabrasive wear was 0.2 mm³ or less was judged to be excellent in wearresistance.

[High-Temperature Oxidation Test]

A specimen having a thickness of 2 mm, a width of 10 mm, and a length of15 mm was cut out from the portion of the sample material describedabove, where a coating film was formed. After being degreased, thisspecimen was subjected to an oxidation test in a heating furnace under agas atmosphere of 20% O₂-5% H₂O-bal.N₂ at 1250° C. for 50 hours.Thereafter, each specimen was taken out and cooled to a normaltemperature; then after an oxide scale generated on the specimen wasremoved, a wall thickness reduction was measured at five arbitrarypoints using a micrometer; and thereafter an average thereof wasdetermined and shown in Table 1.

It is noted that the sample material which showed a wall thicknessreduction of 200 μm or less was judged to be excellent in oxidationresistance.

[Heat Crack Test]

After a coating film of 3 mm thickness was formed by overlaying on Ni—Cralloy having a thickness of 20 mm, a width of 70 mm, and a length of 100mm, a specimen including the coating film and having a thickness of 20mm, a width of 40 mm, and a length of 40 mm was cut out. With theoperation of inserting the specimen into a heating furnace of 1250° C.,keeping it for one hour, and thereafter cooling it to a normaltemperature being as one cycle, 10 cycles thereof were conducted.Thereafter, the occurrence of cracking was evaluated by appearanceobservation and a penetrating test. The occurrence and non-occurrence ofcracking are shown in Table 1.

TABLE 1 Endo- thermic reac- Wall Sam- Film Carbide tion Build-up Abra-thick- Occurrence or ple thick- volume temper- occurrence sive ness non-num- Chemical composition of alloy powder (mass %) Kinds of ness ratioature timing wear reduc- occurrence of ber C Si Cr Co Balance carbide(mm) (vol %) (° C.) (cycle) (mm³) tion (μm) cracking 1 0.06 0.6 26.572.4 — Cr 2.9 50 1300 600 0.13 40 Non- carbide occurrence 2 0.12 0.625.3 62.5 8Fe, 3Mo Cr 3.0 50 1275 600 0.11 50 Non- carbide occurrence 30.47 0.7 24.8 61.6 3.9W, 0.3Nb, 0.1V, Cr 0.7 30 1250 600 0.18 80 Non-7.9Ni carbide occurrence 4 0.11 1.4 26.1 56.1 1.6Mn, 0.2B, 6.2Mo, Cr 2.950 1260 700 0.17 70 Non- 7.9Fe carbide occurrence 5 0.12 0.6 26.2 64.51.5Cu, 6.5Mo, 03Ti, Cr 1.8 70 1290 800 0.09 60 Non- 0.1Al carbideoccurrence 6 0.15 0.2 22.5 65.4 1.5Ta, 0.2Zr, 0.03REM, Cr 4.5 50 1300500 0.12 120  Non- 4.2Fe, 5.5Ni, 0.04Ca carbide occurrence 7 0.01* 1.725.4 68.3 2.9Mo, 1.2Fe Cr 3.0 50 1325  100# 0.32# 90 Non- carbideoccurrence 8 0.75* 1.1 28.2 61.2 4.5W, 2.5Ni, 1.3Fe Cr 3.0 60  1240* 200# 0.08 70 Occurrence# carbide 9 0.12 0.5 25.8 49.4* 0.7Mn, 4.5Mo,9.5Fe, Cr 2.9 50 1270  200# 0.30# 60 Non- 9.2Ni carbide occurrence 100.33 1.4 18.5* 70.4 5.5W, 1.2Mo, 2.2Ni Cr 3.0 50 1272 500 0.11 320# Non-carbide occurrence 11 0.22 0.1* 26.1 68.7 4.5Mo Cr 2.9 50 1315 500 0.12260# Non- carbide occurrence 12 0.57 3.8* 24.5 61.2 4.3Mo, 5.4Ni Cr 3.050  1205*  200# 0.33# 50 Occurrence# carbide 13 0.12 0.6 25.9 62.12.9Mo, 7.5Fe, 0.5Ni, Cr 2.8  15* 1310  300# 0.34# 60 Non- 0.01B carbideoccurrence 14 0.25 1.5 26.7 61.1 5.5Mo, 2.2Fe, 2.5Ni Nb 3.0 50 1380 100# 0.08 >1000#  Non- carbide* occurrence 15 0.25 1.5 26.7 61.1 5.5Mo,2.2Fe, 2.5Ni None* 2.9 50 1330  100# 0.42# 70 Non- occurrence *meansthat the value is out of the range defined by the present invention.#means that required performance is not satisfied.

It is noted that the “film thickness” in Table 1 shows the coating filmthickness of the specimen used in the build-up test and thehigh-temperature oxidation test. The coating film thicknesses of thespecimens for heat crack test were all 3 mm.

As shown in Table 1, in all of Sample No. 7 having a low C content inthe alloy powder, Sample No. 8 having a high C content and a lowendothermic reaction temperature, Sample No. 9 having a low Co content,Sample No. 12 having a high Si content and a low endothermic reactiontemperature, Sample No. 14 using a Nb carbide powder as the hardparticles, and Sample No. 15 including an overlay coating film formedonly with Co-based alloy powder, a build-up occurred in a short timeperiod of 200 cycles or less, indicating poor build-up resistance. Abuild-up occurred at 300 cycles in Sample No. 13 in which the volumeratio of Cr carbide was low, showing poor build-up resistance. In all ofSample No. 7 having a low C content in the alloy powder, Sample No. 9having a low Co content, Sample No. 12 having a high Si content, SampleNo. 13 having a low volume ratio of Cr carbide, and Sample No. 15including an overlay coating film formed with a Co-based alloy powderalone, the abrasive wear was large showing poor wear resistance.

In all of Sample No. 10 having a low Cr content in the alloy powder,Sample No. 11 having a low Si content, and Sample No. 14 using Nbcarbide, the wall thickness reduction was large in the high-temperatureoxidation test, indicating poor oxidation resistance.

Further, in Sample No. 8 having a high C content and Sample No. 12having a high Si content and a low endothermic reaction temperature,cracking occurred in the heat crack test, indicating poor heat crackingresistance.

On the other hand, Sample Nos. 1 to 6 of the present invention wereexcellent in all of build-up resistance, wear resistance, oxidationresistance and heat cracking resistance.

Although only some exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention.

INDUSTRIAL APPLICABILITY

According to the present invention, even if a high-temperature materialis transferred under a high-temperature environment such as inside aheat treatment furnace, no build-up will be produced for a long periodof time, and thus no indentation defect on the transferred material willoccur. Further, it also provides an excellent durability. Thus, sincethe present invention contributes to quality improvement of hot workingproducts, increase of yields, as well as reduction of production cost byextending the life of transfer rollers, advantageous effects thereof areremarkable.

REFERENCE SIGNS LIST

-   1: Cylindrical test material A (Test material that simulates a    high-temperature material transferring member)-   2: Coating film-   3: Cylindrical test material B (Test material that simulates a    material to be transferred)-   4: High-frequency heating coil-   5 a: Test material support (with a rotating and lifting mechanism)-   5 b: Test material support-   6: Heating chamber

1. A high-temperature material transferring member including a coatingfilm formed on a surface of base metal by a plasma powder overlayingprocess, wherein the coating film is a composite coating film using amixed powder made up of a Co-based alloy powder containing, in mass %,0.03 to 0.6% of C, 0.2 to 3% of Si, 22 to 35% of Cr, and more than 50%of Co, and a Cr carbide powder.
 2. A high-temperature materialtransferring member including a coating film formed on a surface of basemetal by a plasma powder overlaying process, wherein the coating film isa composite coating film using a mixed powder made up of: a Co-basedalloy powder containing, in mass %, 0.03 to 0.6% of C, 0.2 to 3% of Si,22 to 35% of Cr, and more than 50% of Co, with the balance beingimpurities; and a Cr carbide powder.
 3. The high-temperature materialtransferring member according to claim 2, wherein the Co-based alloypowder further contains, in mass %, one or more elements selected fromthe following groups <1> to <4>: <1> Mn: 10% or less, Cu: 10% or less,Ni: 10% or less, and Fe: 10% or less; <2> Mo: 10% or less, and W: 10% orless; <3> B: 3% or less, Ti: 3% or less, V: 3% or less, Zr: 3% or less,Nb: 3% or less, and Ta: 3% or less; and <4> Al: 1% or less, Ca: 1% orless, and REM: 1% or less.
 4. The high-temperature material transferringmember according to claim 1, wherein the content of Cr carbide powder is20 to 70 vol % in a volume ratio with respect to a total amount of mixedpowder.
 5. The high-temperature material transferring member accordingto claim 2, wherein the content of Cr carbide powder is 20 to 70 vol %in a volume ratio with respect to a total amount of mixed powder.
 6. Thehigh-temperature material transferring member according to claim 3,wherein the content of Cr carbide powder is 20 to 70 vol % in a volumeratio with respect to a total amount of mixed powder.
 7. Thehigh-temperature material transferring member according to claim 1,wherein an endothermic reaction temperature of the coating film is 1250°C. or more.
 8. The high-temperature material transferring memberaccording to claim 2, wherein an endothermic reaction temperature of thecoating film is 1250° C. or more.
 9. The high-temperature materialtransferring member according to claim 3, wherein an endothermicreaction temperature of the coating film is 1250° C. or more.
 10. Thehigh-temperature material transferring member according to claim 4,wherein an endothermic reaction temperature of the coating film is 1250°C. or more.
 11. The high-temperature material transferring memberaccording to claim 5, wherein an endothermic reaction temperature of thecoating film is 1250° C. or more.
 12. The high-temperature materialtransferring member according to claim 6, wherein an endothermicreaction temperature of the coating film is 1250° C. or more.
 13. Thehigh-temperature material transferring member according to claim 2,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 14. Thehigh-temperature material transferring member according to claim 3,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 15. Thehigh-temperature material transferring member according to claim 5,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 16. Thehigh-temperature material transferring member according to claim 6,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 17. Thehigh-temperature material transferring member according to claim 8,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 18. Thehigh-temperature material transferring member according to claim 9,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 19. Thehigh-temperature material transferring member according to claim 11,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.
 20. Thehigh-temperature material transferring member according to claim 12,wherein the high-temperature material transferring member has excellentbuild-up resistance in a gas atmosphere of 1100° C. or more.